The lithium secondary battery in which a negative electrode is formed by using a material capable of absorbing and discharging lithium ions is capable of restraining deposit of dendrite to a higher extent than a lithium secondary battery in which the negative electrode is formed by using metallic lithium. Therefore the lithium secondary battery has been supplied to the market as a secondary battery having enhanced safety.
In recent years, the development of the lithium secondary battery is advanced to mount it on a vehicle. It is a big problem to improve the performances of the lithium secondary battery so that it can be charged and discharged at a high current and has a high capacity and a long life, even though it is repeatingly charged and discharged.
To overcome this problem, efforts have been made to improve the performance of the lithium secondary battery. Not only a positive-electrode material composed of lithium metal oxide, but also a positive-electrode material containing insulating oxide and a positive-electrode material having a high resistance have been investigated. Investigations have been made on negative-electrodes composed of materials such as a carbon-based material, a material containing titanium oxide, and an alloy-based material which have a high capacity and through which a high current can be flowed.
Efforts have been made to decrease the current density load of the lithium secondary battery by increasing the specific surface area of active substance particles composing the positive-electrode and negative-electrode materials such that the diameter thereof are decreased and by so designing electrodes as to increase the area of the electrodes.
The above-described devices have improved the performance of the lithium secondary battery in allowing the lithium secondary battery to have a high capacity and be charged and discharged at a high current, but were insufficient as a measure for prolonging the life of the lithium secondary battery. Therefore the substitution mixing ratio of metal elements of lithium metal oxides used to form the positive electrode and substitution of doped metals have been investigated. There has been proposed an additive devised to prevent a resistant film from being generated by the decomposition of an electrolyte at the negative electrode composed of a carbon-based material. To improve the performance of a negative electrode composed of an alloy-based material having a semiconductor property, there has been also proposed an alloy composition, the addition of a conductive material, and a binding agent devised to restrain the volume expansion of an alloy. For example, the electrode for a secondary battery composed of the electrode material containing the active substance powder, the conductive material which is formed from the carbon material and attaches to the surface of the active substance powder, and the fibrous conductive material combined with the conductive material is known (patent document 1).
Although the above-described proposed means are capable of increasing the cycle life up to 3000 to 4000 cycle level from hundreds of cycles, the means are insufficient for increasing the cycle life not less than 5000 cycles for not less than 10 years. Thus batteries having the electrodes proposed as described above do not have performance to such an extent that they can be mounted on a vehicle. In the proposal of relaxing the volume expansion of the alloy to increase the adhesive force of the binding agent by using polyimide as the binding agent of the negative electrode composed of the alloy, there is an increase in the use amount of the binding agent when it is used as the technique of making the alloy active substance fine and preventing the alloy active substance from separating from the electricity collection foil, a designed capacity is not satisfied, and the cost increases. Thus it is difficult for the proposed means to satisfy the demanded performance to such an extent that batteries having the electrodes can be mounted on a vehicle.
It is difficult for the above-described proposed means to increase the capacity and output and improve the life of the batteries using the lithium metal oxide having a high electric resistance for the positive electrode, the titanium oxide having the insulating and semiconductor property for the negative electrode, the alloy for the negative electrode, and the carbon-based material having conductivity but a high contact resistance because of particles made fine.